The invention generally relates to an oxidizer for a fuel cell system.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a polymer electrolyte membrane (PEM), often called a proton exchange membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations:H2→2H++2e− at the anode of the cell, and  Equation 1O2+4H++4e−→2H2O at the cathode of the cell  Equation 2
The anode and cathode of a typical PEM fuel cell are formed by locating a catalyst in close physical contact with each side of the PEM. An electrically conductive gas diffusion layer (GDL) is often situated on top of each anode and cathode to improve gas distribution and contact with the anode and cathode catalysts. This combined package of PEM, anode catalyst, cathode catalyst, and GDL is often referred to as a membrane electrode assembly (MEA).
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several MEAs (each one being associated with a particular fuel cell) may be dispersed throughout the stack of flow plates which form the fuel cell stack. Reactant gases from each side of the MEA may leave the flow channels and diffuse through the GDLs to reach the PEM.
The fuel cell stack is one out of many components of a typical fuel cell system, as the fuel cell system includes various other components and subsystems, such as a cooling subsystem, a cell voltage monitoring subsystem, a control subsystem, a power conditioning subsystem, etc. The particular design of each of these subsystems is a function of the application that the fuel cell system serves. Many fuel cell systems include a reformer which reacts hydrocarbon fuel, steam, and often air to form a hydrogen rich gas stream, called reformate, which is the fuel used by the fuel cell stack. Reformate often contains small concentrations of hydrocarbons that passed through the reformer without reacting.
The fuel cell stack produces anode exhaust gas (or “anode tail gas”), a gas that may contain hydrogen that was not consumed in the electrochemical reactions inside the fuel cell stack. The anode exhaust may also contain unreacted hydrocarbons from the reformer. Anode tail gas may be routed to a device, such as an oxidizer, that removes the hydrogen and other hydrocarbons from the flow before venting this flow to the atmosphere. More specifically, the oxidizer reacts any residual hydrogen and/or hydrocarbon fuel that is present in the anode exhaust gas with an oxidant for purposes of removing the hydrogen and/or hydrocarbon fuel and for purposes of recovering thermal energy. Recovering all the thermal energy and minimizing the release of harmful compounds into the atmosphere requires complete, or nearly complete, oxidation of the hydrogen and any hydrocarbons present in the anode tail gas. The reactions between the hydrogen and the oxidant and the hydrocarbon fuel and the oxidant may require a relatively high temperature (a temperature above 600° C., for example).
The anode tail gas produced by some fuel cell systems has a heating value which is too low to support a conventional flame. For this reason, some fuel cell systems use a catalytic anode tail gas oxidizer to promote the oxidation of hydrogen and hydrocarbon fuel at lower temperatures than found in a flame. Catalytic tail gas oxidation typically requires premixing of the anode tail gas and the oxidant to achieve complete oxidation of the hydrogen and the hydrocarbon fuel. Some fuel cell systems also produce anode tail gas with a time-varying composition, due to changes in operating conditions and electrical load on the fuel cell system. Variations in the anode tail gas composition can result in the premixed anode tail gas and oxidant mixture sometimes falling within the flammable regime.
Hydrogen has a relatively high flame speed, which means that when the hydrogen is reacted with an oxidant, and the mixture falls within the flammable regime, the reaction front may easily travel upstream to produce flashback. A device called a flame arrestor may be used with an oxidizer in an attempt to prevent the flashback. However, the use of the flame arrestor typically increases the cost, pressure drop, and complexity of the fuel cell system. In addition, flame arrestors cannot prevent autoignition of a flammable, premixed fuel and oxidant mixture.
Thus, there is a continuing need for a premixed, catalytic oxidizer that achieves complete combustion of the fuel and prevents flashback.